非懒加载类和懒加载类
总纲领: OC底层探寻
上篇: iOS dyld与objc的关联
我们都知道iOS的整体机制是懒加载, 也是就是使用到, 再去加载, 不使用就释放掉来节省内存.
1. 懒加载类
懒加载类, 他的数据加载是推迟到第一次消息的时候:
数据执行的顺序为 lookUpImpForward -> realizeClassMayBeSwifeMayBeRelock -> realizeClassWithoutSwife -> methodizeClass
2. 非懒加载类
map_images的时候, 加载所有数据. (例如: 可以在类中增加 +load()方法, 来将该类变为非懒加载类, 并且我们知道类中load方法在main函数之前调用)
数据执行的顺序为 _getObjc2NonlazyClassList -> readClass -> realizeClassWithoutSwife -> methodizeClass
_read_images
以下是_read_images部分代码, 具体解析请看代码中注释:
// Category discovery MUST BE Late to avoid potential races
// when other threads call the new category code before
// this thread finishes its fixups.
// +load handled by prepare_load_methods()
// Realize non-lazy classes (for +load methods and static instances) - 懒加载类 -> 非懒加载类
// 懒 别人不懂我 我就不动 - 让它 提前加载 - load_images 类
// 懒加载类在什么时候?
for (EACH_HEADER) {
classref_t const *classlist =
_getObjc2NonlazyClassList(hi, &count);
for (i = 0; i < count; i++) {
Class cls = remapClass(classlist[i]);
const char *mangledName = cls->mangledName();
const char *LGPersonName = "LGPerson";
//对研究对象进行过滤 (针对自己写的自定义类进行研究)
if (strcmp(mangledName, LGPersonName) == 0) {
auto kc_ro = (const class_ro_t *)cls->data();
printf("_getObjc2NonlazyClassList: 这个是我要研究的 %s \n",LGPersonName);
}
if (!cls) continue;
addClassTableEntry(cls);
if (cls->isSwiftStable()) {
if (cls->swiftMetadataInitializer()) {
_objc_fatal("Swift class %s with a metadata initializer "
"is not allowed to be non-lazy",
cls->nameForLogging());
}
// fixme also disallow relocatable classes
// We can't disallow all Swift classes because of
// classes like Swift.__EmptyArrayStorage
} // alloc init - 类存在 完备 实例
realizeClassWithoutSwift(cls, nil);
}
}
以下是realizeClassWithoutSwift源码, 具体解析请看代码中注释:
/***********************************************************************
* realizeClassWithoutSwift
* Performs first-time initialization on class cls,
* including allocating its read-write data.
* Does not perform any Swift-side initialization.
* Returns the real class structure for the class.
* Locking: runtimeLock must be write-locked by the caller
**********************************************************************/
static Class realizeClassWithoutSwift(Class cls, Class previously)
{
runtimeLock.assertLocked();
class_rw_t *rw;
Class supercls;
Class metacls;
const char *mangledName = cls->mangledName();
const char *LGPersonName = "LGPerson";
//对研究对象进行过滤 (针对自己写的自定义类进行研究)
if (strcmp(mangledName, LGPersonName) == 0) {
auto kc_ro = (const class_ro_t *)cls->data();
auto kc_isMeta = kc_ro->flags & RO_META;
if (!kc_isMeta) {
printf("%s: 这个是我要研究的 %s \n",__func__,LGPersonName);
}
}
if (!cls) return nil;
if (cls->isRealized()) return cls;
ASSERT(cls == remapClass(cls));
// fixme verify class is not in an un-dlopened part of the shared cache?
/ ro (read only) rw (read write) rwe (read write ext) rwe只在该类有分类时才有意义
// data() -> ro
auto ro = (const class_ro_t *)cls->data();
auto isMeta = ro->flags & RO_META;
if (ro->flags & RO_FUTURE) {
// This was a future class. rw data is already allocated.
rw = cls->data();
ro = cls->data()->ro();
ASSERT(!isMeta);
cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
} else {
// Normal class. Allocate writeable class data.
rw = objc::zalloc();
rw->set_ro(ro);
rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
cls->setData(rw);
}
#if FAST_CACHE_META
if (isMeta) cls->cache.setBit(FAST_CACHE_META);
#endif
// Choose an index for this class.
// Sets cls->instancesRequireRawIsa if indexes no more indexes are available
cls->chooseClassArrayIndex();
if (PrintConnecting) {
_objc_inform("CLASS: realizing class '%s'%s %p %p #%u %s%s",
cls->nameForLogging(), isMeta ? " (meta)" : "",
(void*)cls, ro, cls->classArrayIndex(),
cls->isSwiftStable() ? "(swift)" : "",
cls->isSwiftLegacy() ? "(pre-stable swift)" : "");
}
// Realize superclass and metaclass, if they aren't already.
// This needs to be done after RW_REALIZED is set above, for root classes.
// This needs to be done after class index is chosen, for root metaclasses.
// This assumes that none of those classes have Swift contents,
// or that Swift's initializers have already been called.
// fixme that assumption will be wrong if we add support
// for ObjC subclasses of Swift classes.
// cls 信息 -> 父类 -> 元类 : cls LGPerson
supercls = realizeClassWithoutSwift(remapClass(cls->superclass), nil);
metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);
#if SUPPORT_NONPOINTER_ISA
if (isMeta) {
// Metaclasses do not need any features from non pointer ISA
// This allows for a faspath for classes in objc_retain/objc_release.
cls->setInstancesRequireRawIsa();
} else {
// Disable non-pointer isa for some classes and/or platforms.
// Set instancesRequireRawIsa.
bool instancesRequireRawIsa = cls->instancesRequireRawIsa();
bool rawIsaIsInherited = false;
static bool hackedDispatch = false;
if (DisableNonpointerIsa) {
// Non-pointer isa disabled by environment or app SDK version
instancesRequireRawIsa = true;
}
else if (!hackedDispatch && 0 == strcmp(ro->name, "OS_object"))
{
// hack for libdispatch et al - isa also acts as vtable pointer
hackedDispatch = true;
instancesRequireRawIsa = true;
}
else if (supercls && supercls->superclass &&
supercls->instancesRequireRawIsa())
{
// This is also propagated by addSubclass()
// but nonpointer isa setup needs it earlier.
// Special case: instancesRequireRawIsa does not propagate
// from root class to root metaclass
instancesRequireRawIsa = true;
rawIsaIsInherited = true;
}
if (instancesRequireRawIsa) {
cls->setInstancesRequireRawIsaRecursively(rawIsaIsInherited);
}
}
// SUPPORT_NONPOINTER_ISA
#endif
// Update superclass and metaclass in case of remapping
cls->superclass = supercls;
cls->initClassIsa(metacls);
// Reconcile instance variable offsets / layout.
// This may reallocate class_ro_t, updating our ro variable.
if (supercls && !isMeta) reconcileInstanceVariables(cls, supercls, ro);
// Set fastInstanceSize if it wasn't set already.
cls->setInstanceSize(ro->instanceSize);
// Copy some flags from ro to rw
if (ro->flags & RO_HAS_CXX_STRUCTORS) {
cls->setHasCxxDtor();
if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
cls->setHasCxxCtor();
}
}
// Propagate the associated objects forbidden flag from ro or from
// the superclass.
if ((ro->flags & RO_FORBIDS_ASSOCIATED_OBJECTS) ||
(supercls && supercls->forbidsAssociatedObjects()))
{
rw->flags |= RW_FORBIDS_ASSOCIATED_OBJECTS;
}
// Connect this class to its superclass's subclass lists
if (supercls) {
addSubclass(supercls, cls);
} else {
addRootClass(cls);
}
// Attach categories - 分类
methodizeClass(cls, previously);
return cls;
}
以下为methodizeClass源码, 具体解析请看代码中注释:
/***********************************************************************
* methodizeClass
* Fixes up cls's method list, protocol list, and property list.
* Attaches any outstanding categories.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
// realizeClassWithoutSwift 尽管看到了 data() -> ro -> rw (rwe)
// ro - methodlist - 方法查找的时候 (二分查找) sel 排序 (以下代码为类加载的时候, 就对methedlist进行了排序, 这也就解释了为什么可以用二分查找)
static void methodizeClass(Class cls, Class previously)
{
runtimeLock.assertLocked();
bool isMeta = cls->isMetaClass();
auto rw = cls->data();
auto ro = rw->ro();
auto rwe = rw->ext();
const char *mangledName = cls->mangledName();
const char *LGPersonName = "LGPerson";
//对研究对象进行过滤 (针对自己写的自定义类进行研究)
if (strcmp(mangledName, LGPersonName) == 0) {
bool kc_isMeta = cls->isMetaClass();
auto kc_rw = cls->data();
auto kc_ro = kc_rw->ro();
if (!kc_isMeta) {
printf("%s: 这个是我要研究的 %s \n",__func__,LGPersonName);
}
}
// Methodizing for the first time
if (PrintConnecting) {
_objc_inform("CLASS: methodizing class '%s' %s",
cls->nameForLogging(), isMeta ? "(meta)" : "");
}
// Install methods and properties that the class implements itself.
method_list_t *list = ro->baseMethods();
// 如果有list, 则对list进行排序
if (list) {
prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));
if (rwe) rwe->methods.attachLists(&list, 1);
}
property_list_t *proplist = ro->baseProperties;
if (rwe && proplist) {
rwe->properties.attachLists(&proplist, 1);
}
protocol_list_t *protolist = ro->baseProtocols;
if (rwe && protolist) {
rwe->protocols.attachLists(&protolist, 1);
}
// Root classes get bonus method implementations if they don't have
// them already. These apply before category replacements.
if (cls->isRootMetaclass()) {
// root metaclass
addMethod(cls, @selector(initialize), (IMP)&objc_noop_imp, "", NO);
}
// Attach categories.
if (previously) {
if (isMeta) {
objc::unattachedCategories.attachToClass(cls, previously,
ATTACH_METACLASS);
} else {
// When a class relocates, categories with class methods
// may be registered on the class itself rather than on
// the metaclass. Tell attachToClass to look for those.
objc::unattachedCategories.attachToClass(cls, previously,
ATTACH_CLASS_AND_METACLASS);
}
}
objc::unattachedCategories.attachToClass(cls, cls,
isMeta ? ATTACH_METACLASS : ATTACH_CLASS);
#if DEBUG
// Debug: sanity-check all SELs; log method list contents
for (const auto& meth : rw->methods()) {
if (PrintConnecting) {
_objc_inform("METHOD %c[%s %s]", isMeta ? '+' : '-',
cls->nameForLogging(), sel_getName(meth.name));
}
ASSERT(sel_registerName(sel_getName(meth.name)) == meth.name);
}
#endif
}
类扩展VS分类
category : 类别, 分类
- 专门用来给类添加新的方法
- 不能给类添加属性, 添加了成员变量, 也无法取到 (具体要看分类的结构)
- 注意: 其实可以通过runtime给分类添加属性
- 分类中用@property定义变量, 只会生成getter, setter方法的声明, 不能生成方法实现和带下划线的成员变量.
extension : 类扩展
- 可以说是特殊的分类, 也称作匿名分类
- 可以给类添加变量属性, 但是是私有变量
- 可以给类添加方法, 也是私有方法.